The present invention relates to a hydraulic brake system with slip control for automotive vehicles, comprising a master cylinder pressurizable by a hydraulic power booster. The brake system further comprising valve means which are disposed between the master cylinder and the wheel brakes connected to the master cylinder and which are controllable by a slip-monitoring arrangement and by which the wheel brakes are connectable to an unpressurized reservoir or to at least one brake line leading to the outlet of the master cylinder. The brake system further comprising a change-over valve which is controllable by the slip-monitoring arrangement and by which the brake line is connectible with the pressure chamber of the hydraulic power booster.
A hydraulic brake system of this type is known from German printed and published patent application No. 33 38 249. In a braking pressure generator arrangement, this brake system includes a tandem master cylinder which is actuated by the hydraulic power booster. By virtue of a brake valve, the hydraulic power booster sets the pressure in the pressure chamber such that it is proportional to the actuating force exerted on the brake pedal. Allocated to the wheel brakes connected to the working chambers of the tandem master cylinder are at least one pair of one normally opened and one normally closed multidirectional valves. The opened multidirectional valve is situated in the path of flow between the working chamber of the tandem master cylinder and the wheel brake, while pressure fluid can be discharged from the wheel brake through the closed multidirectional valve when the opened multidirectional valve has been switched by a slip-monitoring arrangement to assume its closed position.
Pressure supply of the hydraulic power booster is provided by a huydropneumatic pressure accumulator which is caused to maintain a predetermined pressure level by way of a pressure-fluid pump and a pressure monitor. The change-over valve is a three-way/two position directional valve, the coil of which is applied with operating voltage by way of the slip-monitoring arrangement when a critical slip condition occurs at any one or more of the vehicle wheels. After change-over of the three-way/two-position directional valve, the wheel cylinders are connected by way of valves with the pressure chamber of the hydraulic power booster so that the pressure fluid removed from the wheel brakes during slip control can be replenished out of the pressure chamber of the power booster. Hence it follows that pressurization of the wheel brakes is performed dynamically through central valves connected downstream of the master-cylinder chambers.
It is an object of the present invention to improve upon a hydraulic brake system of the type referred to so that during a braking action after the first response of the slip-mounting arrangement, further actions of pressure reduction and pressure increase in the wheel brakes are controllable by way of the pressure chamber of the hydraulic power booster.